SNT (Suc1-binding Neurotrophic factor-induced Tyrosine-phosphorylated target) proteins are recently discovered adaptor molecules that undergo rapid tyrosine phosphorylation following stimulation by growth factors including neurotrophins (NGF, BDNF, NT3) and FGFs but not by insulin and EGF. They are distantly related to IRS. SNTs are myristoylated and membrane-anchored and bear an N-terminal phosphotyrosine-binding (PTB) domain that mediates weak, but functionally significant SNT-receptor interactions. The ligand- dependent tyrosine phosphorylation of SNTs enables SNTs to interact with at least two different SH2 domain-containing moieties, Grb2/Sos and Shp2. As activation of both the Grb2/Sos and the subsequent Ras activation as well as the Shp2 phosphatase has been shown to be required for a wide-range of FGF- and neurotrophin- induced biological responses, SNTs may be required for activation of two biologically critical pathways. Three aims are proposed in this application to elucidate the biochemical and biological properties of SNTs. Aim 1 is proposed to determine the nature of the interaction between SNT PTB domains by using a combination of NMR and mutagenesis approaches. A possible direct interaction between SNTs and another receptor tyrosine kinase Ret will also be tested. In specific aim 2, the biological function of SNT1 and SNT2 will be addressed. Two approaches will be undertaken for this purpose: 1.) Dominant negative mutants of SNTs will be overexpressed in cultured cells and 2.) Mice with null, hypmerorphs and conditional null mutations in SNT genes will be generated. Finally, in specific aim 3, the idea of SNTs serving as potential biological specificity determinants will be tested. To this end, chimeric SNT-IRS as well as insulin receptor-Trk hybrids will be produced to determine if they can turn insulin response to that of neurotrophins or FGFs.